1. Field of the Invention
The present invention relates to a thermoelectric device and a method of making thereof which make possible electric generation by the temperature difference (thermal power generation) by the Seebeck effect and thermoelectric cooling and heat generation by Peltier effect.
2. Prior Art
A thermoelectric device is made by bonding a P-type thermoelectric material and a N-type thermoelectric material via an electrically conductive electrode such as a metal to thereby form a couple of PN junctions. The thermoelectric device generates thermal electromotive force based on the Seebeck effect by a temperature difference applied between ends of the junction couple. Therefore, it has applications for a power generating device and conversely, a cooling device and a fine temperature control device utilizing the so-called Peltier effect in which one side of a junction is cooled and the other side generates heat by making electric current flow in the device and the like.
Generally, a thermoelectric device is used as a module in which a plurality of couples of PN junctions are connected in series to promote its function. In the structure of this module pieces of P-type and N-type thermoelectric materials (called thermoelectric material chip) having a shape of a rectangular parallelopiped of which size ranges from several hundred .mu.m to several mm are interposed by two sheets of electrically insulative substrates of alumina, aluminium nitride or the like, the P-type thermoelectric material chips and N-type thermoelectric material chips are PN-coupled by electrodes-of an electrically conductive substance such as a metal formed on the substrates and at the same time the thermoelectric material chips are connected in series by these junctions.
FIG. 16 illustrates views showing an arrangement of electrodes of substrates and thermoelectric material chips at a section cut in a direction in parallel with the substrates and respective sections in a direction orthogonal to the substrates of a conventional thermoelectric device (hereinafter called a thermoelectric device including a module in which the above-mentioned plurality of thermoelectric chips are arranged) having such a structure. FIG. 16A is a view showing an arrangement of electrodes and thermoelectric material chips on the substrate at a section in parallel with the substrates of the conventional thermoelectric device. In other words, it is a perspective view for indicating the arrangement of the electrodes and the thermoelectric material chips from above the substrate. An electrode pattern shown by bold lines indicates an electrode 161 of a top substrate whereas an electrode pattern shown by dotted lines indicates an electrode 162 of a bottom substrate. Further, a hatched quadrangle at the inside of a portion in which the electrode 161 of the top substrate intersects with the electrode 162 of the bottom substrate indicates a portion in which a P-type thermoelectric material chip 163 or a N-type thermoelectric material chip 164 is disposed. FIGS. 16B, 16C, 16D are views showing respective longitudinal sections of FIG. 16A taken along lines X1-X1', X2-X2' and Y1-Y1'. As is apparent from FIG. 16, the arrangement of the thermoelectric material chips in the conventional thermoelectric device is in a lattice form arranged on the substrate and the P-type thermoelectric material chips and the N-type thermoelectric material chips are always arranged alternately in respective rows (X direction and Y direction in FIG. 16A) constituting the lattice.
An explanation will be given of a method of making the conventional thermoelectric device comprising a plurality of the thermoelectric material chips as follows.
FIG. 17 illustrates views showing an outline of working thermoelectric material in manufacturing the conventional thermoelectric device by longitudinal sections thereof. FIG. 17A shows a section of a thermoelectric material 171 which has been worked in plate-like form or rod-like form. Layers 172 are formed for soldering by Ni etc. on both faces of the thermoelectric material to be bonded to the substrates by a plating method (FIG. 17B). Next, P-type and N-type thermoelectric material chips 173 each having the layers 172 for soldering on its both faces are formed by cutting the thermoelectric material (FIG. 17C).
Successively, each of the thermoelectric material chips formed as above is disposed on a predetermined electrode on the substrate by using jigs or the like and a bonding is performed thereby forming the thermoelectric device. FIG. 18 illustrates views showing a conventional method of manufacturing a thermoelectric device by using the thermoelectric material chips and substrates provided with electrodes. FIG. 18A shows relationship between the substrates 181 and thermoelectric material chips 182 before bonding. Electrodes 183 for forming PN junctions and bonding materials 184 for bonding the thermoelectric material chips 182 to surfaces of the substrates are formed on the substrates 181 in layers. FIG. 18B shows a longitudinal sectional view in which a thermoelectric device 185 is formed by bonding the respective portions.
Each thermoelectric material chip used for a thermoelectric device is a rectangular parallelopiped having sides with a size ranging from several hundred .mu.m to several mm. However, in recent years, in an device used at around room temperature under a temperature difference of several tens degrees it has high function when its size and thickness ranges from several tens to several hundred .mu.m. For example, such a content is described in, The "Transaction of the Institute of Electronics, Information and Communication Engineers C-II, Vol. J75-C-II, No. 8, pp. 416-424(JAPAN)" (in Japanese) and the like, while importance of design with respect to heat is set forth in the same paper.
Further, the number of couples of thermoelectric material chips in one thermoelectric device has been several hundreds at most and its density has been approximately several tens couples/cm.sup.2. However, to increase the number of couples of thermoelectric material chips is one of very important factors in promoting its function and expanding its application. Especially, in power generation using a small temperature difference, generated electromotive force is in proportion to the number of couples of thermoelectric material chips and therefore, it is desirable to increase as many as possible the number of thermoelectric material chips connected in series in a thermoelectric device to generate a high voltage. Furthermore, also in case where a thermoelectric device is used as a cooling device or a temperature controlling device, electric current flowing in an device is enhanced when the number of thermoelectric material chips connected in series is small and it is necessary to enlarge wirings or to enlarge power sources. Accordingly, it is desirable to arrange as many thermoelectric material chips as possible in series.
As state above, miniaturizing, thinning, thermal design and an increase in the number of the couples of the thermoelectric material chips connected in series in a single thermoelectric device amount to high function of the thermoelectric device and at the same time are becoming points of expanding its application.
However, in making thermoelectric devices having the conventional structure shown in FIG. 16 by the manufacturing method shown in FIG. 17 and FIG. 18, it is necessary to handle the thermoelectric material chips one by one and there is a limitation for reducing the size of chip and the size of the device considering the operational performance and the working accuracy. Especially, thermoelectric materials having good function including Bi--Te series materials, Fe--Si series material and the like are substances having low mechanical strength. Therefore, in making a thermoelectric device in which the size of the thermoelectric material chip is no more than several hundreds .mu.m or the number of chips is extremely large, the handling of the thermoelectric material is difficult and it is difficult to make thermoelectric device having the conventional structure by the conventional manufacturing method.
Further, when a large number of thermoelectric elements are line-up in series with one another, if there is a discontinuity even at just one part of the electrodes or the thermoelectric materials, the function of the device is impaired. This problem lowers the manufacturing yield and at the same time is considered important from the point of view of cost.
It is an object of the present invention to provide a thermoelectric device which is small in scale and high in function and a method of making thereof by reducing the size of the thermoelectric material chips and increasing the number of thermoelectric material chips per unit area (chip density).